There arise applications in digital imaging where one may want to hide one image within another. The foremost example of this is the field of data hiding, where the goal is to make an image more useful by enabling it with the ability to carry digital data as well as the actual image. Data hiding is more properly referred to as steganography from the Greek words steganos meaning covered and graphia meaning writing or representation. Steganography precedes the digital era and was generally a technique where written messages were first in drawings or paintings by obscuring them in the image's texture (such as vines) so that they could be read only if one knew where to look (often using a mirror). Recently, there has been a desire to hide a message in the form of a digital data sequence within an image. In this approach it is necessary to use a computer to read, or extract, the message. As a result, it is unreadable to the human eye. In all the forms of steganography, both antique and digital, the inclusion of the message results in a distortion of the original image. However, the goal is that the resulting distortion is undetectable to the human eye.
The applications where digital steganography is a potential solution include tamper proofing, digital watermark, image tagging, digital pointers, and data augmentation. Tamper proofing determines if a digital image has been manipulated from its original form and is useful for legal evidence and medical applications. The digital watermark establishes ownership and include applications of copyright protection in the form of either a notice or an actual usage prevention (such as digital copier automatically refusing to print an image with digital copyright notice embedded). Image tagging is an application where an owner gives an image or a sequence of images (i.e., a movie) to a number of distributors, and can place an individual tag on each image such that if the image is found, the distributor of the image can be identified. An example use of image tagging is in the prevention of bootlegged video movies.
Data Augmentation refers to hiding a digital message in an image that contains a description of the image analogous to a caption underneath a photo in a newspaper. A digital pointer refers to hiding a digital message whose contents are a pointer to a location such as a database or a site on the Internet where further information about the picture resides. This information can be similar to the idea of data augmentation, but can also be more encompassing. For example, it can include information on how to segment the image into different areas, information to associate with those areas, and other related images such as the same scene from a different view or with subjects having different expressions or even a higher quality version of the same image (e.g., one with higher resolution).
In the digital version of steganography, the desired distortion most often takes the form of a two dimensional pattern (See "Techniques for Data Hiding" by Bender et al. SPIE Proceedings 2420, 1995), and it is desirable to keep the pattern's contrast as high as possible in order to maintain a high signal to noise ratio. Here the first pattern fills the role of signal, and the image fills the role of noise, even though they look much the opposite to the observer. A high signal to noise ratio enables a decoder to extract the full bit capacity of a message contained in the first pattern. On the other hand, a competing desire is to keep the pattern's contrast as low as possible so that is not visible to the human eye in the image. If one tries to use a fixed pattern of a constant contrast, the specific details of the image such as background noise level in the image and image content (e.g. various edges and textures in the image) make it difficult, and generally impossible, to achieve the competing goals of having the decoder be able to detect a message contained in the first pattern while having the pattern be invisible to a human observer. As a result, the bit capacities of the data hiding techniques described by Bender are low and in some portions of images (i.e. uniform areas with low noise) the patterns will be visible.
Another collection of prior art accomplishes data hiding (5,091,966 issued 2/1992 to Bloomberg et al., 5,128,525 issued 7/1992 to Stearns et al., 5,168,147 issued 12/1992 to Bloomberg, 5,221,833 issued 6/1993 to Hecht, and 5,245,165 issued 9/1993 to Zhang), but it is limited to placing the data within uniform fields of color or gray value, or the special case of text characters, which can be considered as small and very controlled uniform fields. None of these approaches address the issue of how to hide a pattern or embed data in an existing photographic image or a computer generated graphic containing complex content.
The object of the present invention is to provide an improved method for hiding one digital image or pattern within another.